Hidden sectors

At the moment I am struggeling to find out about hidden sectors. So far I've been unable to find any in depth explicit discussion about them anywhere, so if someone here could answer some of my questions it would be very useful for me.

On Wikipedia it mentions that hidden sector particles are particles that do not interact with matter via the usual gauge bosons W, Z, gluons and photons. However it does not really elaborate on this, so I'm left wondering what this really means. Are hidden sector particles essentially particles residing on some other dimesion? Any general explanation would be great.

What type of particles exist in hidden sectors? Are there any limitations as to what particles are allowed to be hidden? I have heard an explanation concerning how we - in the ordinary sector - would observe the masses of hidden sector particles, but this invloved things like the distance of the particles from the intersection of extra-dimensional branes, which is waay over my head! So again any explanation about how we would, in theory, observe hidden sector masses (i.e. would they be large or small etc) would be good.

Also, I'm interested in why people are interested in hidden sectors. Is it simply because they are invloved in extensions of the standard model, and so explore new physics?

Hi there, Mr. Hyde. I'm not an expert in this, so I'll necessarily be a little hand-wavy... but I'll do my best to get the flavor of the idea as I understand it.

Part of the reason why it's hard to find a good explanation for "hidden sectors" are that they're a fairly loosely defined tool used in various ways; i.e. the phrase doesn't really refer to any particular class of models, but rather to a feature that many models require.

In a sentence, a hidden sector is a set of particles that are decoupled (or only weakly coupled) to the Standard Model. This means that it's really hard for us---being composed of Standard Model particles---to detect them through direct interaction with the Standard Model fields. If the hidden sector completely decouples from the Standard Model, then from a strictly `particle physics point of view' they might as well not exist because they don't interact with us at all.

But even if the hidden sector Lagrangian has no coupling to the Standard Model, we would still generically expect the hidden sector to still couple to gravity (or else it would be *very* strange). Thus one could expect that at the very least we could have hints of the existence of the hidden sector through its gravitational effects. Thus one can already see an application for models with a hidden sector: dark matter. It's possible that there is a hidden sector containing very heavy particles that constitute dark matter. The hidden sector fields would not couple (or would couple only very weakly) to the Standard Model, hence they would not interact with photons ('dark') and would evade collider signatures by virtue of their mass or their weak coupling.

At this point I should probably point out that the 'hidden' really only refers to the fact that the Standard Model has a hard time interacting with this sector. This doesn't mean that the 'hidden sector' is physically hidden away from us via an extra dimension or something like that (though this is possible, see below). Indeed, if dark matter is primarily a hidden sector field, then (at least for 4D models) it exists in the same spacetime as the Standard Model -- the SM fields and hidden fields overlap, they just don't couple to each other much so they don't 'feel' each other. For a pop-culture reference, the hidden sector fields in this case are something like the ghosts in the movie "The 6th Sense" -- they exist along with the living, but can only communicate with the living through that little kid and otherwise don't interact with us at all.

What lives in a hidden sector? It could be anything. Heck, the hidden sector could contain its own version of the Standard Model with its own sentient life forms who are pondering the hypothetical existence of our sector. Usually in models of new physics people aren't too precise about the exact field content of the hidden sector because we usually only care about how the hidden sector affects our visible sector (i.e. Standard Model). This is where hidden sectors have some utility. For example, if we have a symmetry that is broken weakly, one could conjecture that such a symmetry is respected by our sector but broken strongly in the hidden sector. The weak coupling between our sectors would then 'transmit' this breaking, but only weakly. It doesn't matter how it's broken in the hidden sector or what kinds of fields there are... only that there is a weak coupling to otherwise-hidden fields that can violate the symmetry and translate that symmetry violation back to us. A good example of this is supersymmetry, which I'll mention below.

You mentioned mass scales of hidden sectors. We usually talk about hidden sectors being very massive. Thus the 'hiding' mechanism is the statement that whatever force couples the Standard Model to the hidden sector (the "messenger") is carried by a very massive particle (e.g. a heavy gauge boson). Since the messenger particle is so massive any amplitude coupling the two sectors is suppressed by inverse powers of the messenger mass. However, there has also been some interest in hidden sectors that are relatively light. These hidden sectors must still have some very heavy messenger or other mechanism to sequester it from the Standard Model to explain why we haven't seen these light particles, but these have been used to describe non-trivial experimental signatures. (The main reference here are the 'Hidden Valley' models by Strassler et al.)

The discovery of a hidden sector would really depend on how well we can probe the 'messenger sector,' i.e. whatever mechanism couples it to our sector.

There are a couple of notable examples of hidden sectors in the two major 'beyond the standard model' paradigms: extra dimensions and supersymmetry.

In braneworld extra dimensional models such as the Randall-Sundrum warped space model, the Standard Model is localized (more or less) on or near a 4-dimensional spacetime called a brane. In principle there could be other branes that contain their own localized fields. The two branes would be coupled gravitationally, but could also couple to 'bulk fields' with can propagate in the extra dimension. In this sense the hidden sector is actually physically separated from our 4D spacetime.

In supersymmetry the big question is why isn't Nature supersymmetric at the scale of the Standard Model? We assume that SUSY must then be broken at some higher scale, e.g. the TeV scale. Mechanisms for breaking SUSY spontaneously at this scale are rather restricted, so the most common solution is that SUSY is broken explicitly by some hidden sector and this breaking is transmitted to our sector via some messenger sector. The type of messenger sector specifies a lot about the SUSY breaking pattern (e.g. gauge mediation, anomaly mediation, gravity mediation).